Acryl/melamine-based resins have been widely used as automotive top clear coatings because of their excellent performance in terms of weather resistance and attractiveness. However, acryl/melamine-based resins have poor acid resistance because they employ melamine resins as curing agents and, when applied as paints the coatings become stained by acidic rain, thus impairing the appearance.
In order to overcome this problem, new curing systems have been desired to replace melamine and therefore much research is being carried out on curing systems which utilize crosslinking reactions between acid groups and epoxy groups. For example, in Japanese Unexamined Patent Publication No. 63-84674 there is proposed a high solid-content curing composition which contains a low molecular weight polyepoxide, a low molecular weight hydroxyl group-containing polyfunctional substance, a crosslinking agent comprising an acid anhydride, and a curing catalyst, as a paint composition with excellent adhesion, luster and sharpness. Also, Japanese Unexamined Patent Publication No. 1-139653 proposes a thermosetting solvent-type coating composition which comprises an acrylic copolymer with acid groups and an acrylic copolymer with epoxy groups, as a coating composition with excellent acid resistance, solvent resistance, moisture resistance and coating appearance.
Nevertheless, the use of crosslinking agents containing acid anhydrides as disclosed in Japanese Unexamined Patent Publication No. 63-84674 is associated with problems in that the increased reactivity between the composition components results in poor storage stability, thus making it difficult to use the composition components in a single solution and reducing handleability. Also, thermosetting solvent-type coating compositions such as disclosed in Japanese Unexamined Patent Publication No. 1-139653 have poor low-temperature curing properties, and thus when baked at the relatively low temperatures employed in automobile baking lines, they have the disadvantage of exhibiting inadequate coating performance, including poor crack resistance, poor acid resistance and poor solvent resistance, in temperature variable environments.
For the purpose of solving these problems, Japanese Unexamined Patent Publication Nos. 2-45577, 3-287650 and 4-363374 have proposed thermosetting compositions containing copolymers with acid anhydride groups in the form of half-esters as the acid groups, with hydroxy compounds and epoxy compounds, as well as thermosetting compositions containing copolymers with acid anhydride groups in the form of half-esters as acid groups, and compounds with epoxy groups and hydroxy groups. However, since these thermosetting compositions have high contents of monomer units with half-esterified acid anhydride groups, they are still unsatisfactory from the standpoint of storage stability, while their lack of monomer units with acid anhydride groups presents the same disadvantage of insufficient low-temperature curing properties and poor solvent resistance.